This invention relates to in situ recovery of shale oil, and more particularly, to techniques for operation of an in situ oil shale retort formed with a void between a top boundary of unfragmented formation and the fragmented permeable mass of oil shale particles in the retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen," which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the change of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595, 4,043,596; 4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a substerranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. No. 4,043,598 discloses a method for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to a method disclosed in that patent, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone into the voids on either side of each unfragmented zone to form a fragmented mass having a void volume equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
It is desirable to have a generally uniformly distributed void volume, or a fragmented mass of generally uniform permeability so that oxygen supplying gas can flow relatively uniformly through the fragmented mass during retorting operations. Techniques used for explosively expanding zones of unfragmented formation toward the horizontal free faces of formation adjacent the voids can control the permeability of the fragmented mass. A fragmented mass having generally uniform permeability in horizontal planes across the fragmented mass avoids bypassing portions of the fragmented mass by retorting gas as can occur if there is gas channeling through the mass owing to non-uniform permeability.
It is desirable to establish a primary combustion zone which is relatively flat and extends laterally across the entire fragmented permeable mass. The relatively flat primary combustion zone extending laterally across the entire fragmented permeable mass enables retorting of a maximum volume of fragmented permeable mass of oil shale which tends to maximize the yield of liquid and gaseous products from the in situ oil shale retort. A flat primary combustion zone extending laterally across the entire fragmented permeable mass can be created by using a burner to ignite a portion of the fragmented permeable mass and introducing a fuel and an oxygen containing gas into the retort to spread the primary combustion zone laterally.
The burner is ignited and the flame from the burner is directed downwardly toward the fragmented permeable mass to heat an upper portion of the fragmented permeable mass to greater than the self ignition temperature of carbonaceous material in the oil shale. The burner is then withdrawn from the retort and fuel and an oxygen containing gas are introduced for establishing a secondary combustion zone, thereby spreading the primary combustion zone laterally. Some difficulty has been encountered in causing a primary combustion zone to spread laterally near the top of an in situ oil shale retort where the fragmented mass completely fills the retort cavity. Thus, considerable time and fuel can be consumed in causing the combustion zone to spread laterally to a sufficient extent to recover shale oil from some of the upper portions of the fragmented mass. Previous techniques have caused the combustion zone to propagate downwardly a considerable distance as it spreads laterally, the downward and radial distance being about the same. This can cause some of the shale oil produced in the upper portions of the fragmented permeable mass to pass through downwstream portions of the primary combustion zone thereby being consumed. The consumption of shale oil or the complete bypassing of portions of the oil shale in the upper corners of the retort tends to reduce the yield of shale oil from the retorting operation. It is therefore desirable to have a technique for improving the rate and extent of lateral spreading of the combustion zone near the top of the retort.
The rate and extent of lateral spreading of the primary combustion zone can be improved by using a retort which has a void between the top boundary of unfragmented formation and the upper surface of the fragmented permeable mass. This void acts as a plenum which permits uniform gas distribution over the top of the fragmented permeable mass and enhances the lateral propagation of the combustion zone.
U.S. Pat. No. 4,027,917 issued to W. J. Bartel et al and assigned to the same assignee as the present invention, discloses a method of igniting an oil shale retort by directing a combustible gas inlet mixture into an ignition zone extending across the top of the in situ oil shale retort. Although the ignition zone has a high void volume compared to the average void volume of the retort, the ignition zone is completely filled with fragmented oil shale.
The creation of an in situ oil shale retort with a void across substantially the entire upper surface of the fragmented permeable mass between the top boundary of unfragmented formation and fragmented permeable mass can be accomplished by detonation of explosive in a time delay sequence which is designed to produce a fragmented permeable mass of generally uniform permeability.
The use of in situ oil shale retort with such a void can result in mining problems after the completion of retorting. Unless the void is substantially completely filled after the completion of the retorting, the overlying formation will remain unsupported, possibly causing the surface to subside creating safety hazards in the area. Further, where there is no direct support for overlying formation, stresses can build up in the formation surrounding the void. These stresses can cause overloading of the formation in the area of the void which can result in fracturing and/or collapse of the formation during subsequent mining operations in adjacent workings.